Could someone show an example of how psychic powers work in Deathwatch? I am confused because I am used to how they worked in Dark Heresy which is different, the individual powers don't have thresholds anymore, it simply appears to be a willpower test now.

3 Answers
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There's been some important errata to how psy powers work, which is good, because the book's rules are actually somewhere between "unclear" and "contradictory" in several areas.

The most important thing to know is that the example is wrong, wrong, wrong, and should be wholesale disregarded.

The other important thing to know is that all Focus Power tests are made at a difficulty of Challenging (+0).

Also, note that you get +5 to your Focus Power test per point of effective Psy Rating - "effective" meaning "including modifiers for Fettered or Push". (Push provides +3 Psy Rating, and thus +15 to your check.)

Having looked at my Rogue Trader book for an unrelated project, it turns out that this system is apparently identical to the mechanics for psy powers that Astropaths use - and that section has a sidebar about how Dark Heresy psy rules (such as things that affect threshold) interact with Rogue Trader psy rules.
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Burrito Al PastorOct 7 '10 at 23:52

The book example is fairly basic: a librarian (Psy 4) decides to take out an orc with Smite. He chooses to push (power level) giving a modified Psy rating of 7 (push grants a +3). Our librarian makes an Challenging (+0) test against his willpower to hit (per the power's description). If successful, he uses the Psy rating of 7 for all power effects and will make a roll on the psychic phenomena chart and handle fatigue (due to pushing).

If you fetter the power you half your psy rating (round up) so PSY=2, WP=45 therefore FP=45+10=55

If you push you add +3 to your Psy modifier to your psy rating, therefore WP=45 PSY=4+3=7 so FP test = 45 + (5*7) 30= 75 but you roll on psychic phenomena table, if you get a double ie 66 you also get 1 point of fatigue
. A FP roll of 91-100 still fails, regardless of wether your Focus Power bonus is greater.

Note: You can use Invocation (full action) to clear your mind an increase the psy rating of a fettered attack by 1, effectively removing the risk of psychic phenomena. A force staff gives +15 to a Invocation test.